The present application relates generally to gas turbines engines, and more specifically to control systems and a method for controlling a load point of a gas turbine engine.
Environmental regulations require power generation industry to continually lower its emission levels. The emissions most commonly regulated are nitrogen oxides (NOx) and carbon monoxide (CO). NOx emissions are generally associated with relatively a high temperature combustion process, while CO could be related to either a relatively high temperature combustion process (dissociation) or to incomplete combustion which occurs when a temperature of combustion is too low or not enough oxidant (oxygen from air for example) is available in the combustion process. A modern approach to low emissions gas turbine engine combustion is know as DLN (Dry Low NOx) combustion. This technology relies on premixing the fuel and air to form a combustible mixture which is then introduced and burned in a reaction zone of a gas turbine combustor. Low emissions are achieved due to a flame temperature lower than a flame temperature for a non-premixed combustion system (fuel and air mix in the combustion reaction zone) for the same gas turbine engine operating conditions. At nominal operating conditions (base load of the gas turbine engine) a DLN combustor could be designed to achieve relatively low NOx emissions and usually CO is not a concern. As the load of the gas turbine engine is decreased from base load, the temperature of combustion decreases. In this process the NOx emissions from a DLN combustor decrease but below some temperature level CO can exceed permitted levels. Therefore a DLN combustor cannot operate in premixed mode at all load conditions of a gas turbine engine. At low loads a pilot flame can be initiated or a non-premixed mode (diffusion combustion) can be employed. Operation under these conditions results in higher emissions compared to a premixed mode of operation and may be restricted to short period of times (transients during short time periods). Exemplary embodiments described herein teach a way to extend a range of load points for which a DLN combustor can operate in a premix mode.
A load point for a gas turbine engine is defined as a percentage of nominal output generated by the gas turbine engine. A gas turbine combustor turndown is defined as a temperature range within which a combustor portion of a gas turbine engine can operate in a stable manner, without undesirable pressure pulsations while meeting emissions requirements. A gas turbine engine turndown is defined as a lower threshold load point of the gas turbine engine at which the emissions requirements can still be met. The gas turbine engine turndown is limited by a combustion system and by an upper threshold temperature in the gas turbine exhaust duct. Operation at lower load points (i.e. below turndown) is not permitted because some operational boundaries expressed in terms of emissions, dynamics, and exhaust gases temperature are violated.
In order to increase the overall efficiency a gas turbine power plant can be configured and operated in a combined cycle. That means that a relatively high temperature exhaust gas from a gas turbine engine is passed through a heat recuperator steam generator (HRSG) to produce steam that drives a steam turbine. Usually two gas turbines are connected to a steam turbine. Combined cycle operation requires that the exhaust gases leaving the gas turbine engine be within a specific temperature range. That is the exhaust temperature cannot be too high to avoid degrading the gas turbine exhaust duct and HRSG hardware. Further, the temperature should not fall below a certain temperature value to avoid a condition called forced cooling when thermal transients in steam turbine rotor and casing can degrade the turbine rotor. The two temperatures limits discussed above are referred to as the upper threshold isotherm and the lower threshold isotherm respectively.
In order to respond to seasonal and often daily changes in power demand gas turbine engines are often subjected to large sweeps in load. Maximum operation flexibility is achieved for a lowest turndown possible which in turn, as described above is limited by the available technologies (combustion and materials). Profitability models which account for spot energy and fuel prices may require the unit to be shutdown because of insufficient turndown. This strategy requires frequent re-starts which limit the ability of the unit to respond to a sudden energy demand and also reduces an operational life of the gas turbine hardware.